B cell superantigens (B cell SAgs), unlike conventional antigens, bind to the Fab regions of immunoglobulin (Ig) molecules outside their complementarity determining regions. These unconventional antigens can react with a substantial amount of a host's peripheral B cells and serum immunoglobulins by virtue of their ability to interact with many members of an entire variable region heavy (VH) or variable region light (VL)gene family. For example, protein L, secreted by the anaerobic strain Peptostreptococcusmagnus, reacts with the Fabs of most human Vk1+, Vk3+, and Vk4+ Igs and homologous murine Vk+ Igs. The ability of B cell SAgs to bind to a large amount of serum IgG underscores their potential to cause imune complex-mediatedtissue injury. The overall objective of the studies proposed in this application is to elucidate the cellular and molecular pathogenesis of B cell SAg-induced immune complex-mediated inflammation in the lung. The specific goals are to determine whether lung inflammation induced by the B cell SAg, protein L, 1) demonstrates histopathological correlates of immune complex tissue injury, 2) requires the interaction with selective Vk Igs and 3) is orchestrated by cells/proinflammatory mediators involved in conventional antigen/antibody complex-mediated tissue injury. These objectives will be examined by using mouse strains manipulated genetically to isolate the factors of interest. Since most of the B cell SAgs described to date are derived from ubiquitous microbes, information from these studies will likely provide insight into mechanisms by which infectious agents contribute to the pathogenesis of immune complex-mediated rheumatic and autoimmune disorders.